The present invention relates to a digital electronic still camera for converting the image of an object to digital data and recording the digital data in a medium and, more particularly, to a digital electronic still camera operable in a text mode for clearly shooting a bilevel image, e.g., characters printed or formed on a document.
Generally, a digital electronic still camera includes a CCD(Charge Coupled Device) image sensor or similar imaging device for generating an image signal. The image signal has customarily been processed as an analog signal up to a CDS (Correlative Dual Sampling) circuit, then transformed to a digital image signal by an analog-to-digital converter, and then processed in various ways by a digital process. A lens, also included in the camera for focusing the image of an object onto the imaging device, has such a characteristic that the quantity of light incident thereto decreases with an increase in the distance from the optical axis. In the light of this, a camera of the type handling an analog image signal is often provided with a shading correction circuit. The shading correction circuit corrects the decrease in the quantity of light in the peripheral portion of an image which is represented by the analog image signal.
However, when the shading correction circuit is digitized in a digital electronic still camera which processes an image signal as a digital signal, the circuit is scaled up. Particularly, in the case of a miniature camera of this type, the degradation of an image is conspicuous because the diameter of a lens available with the camera is limited. A digital electronic still camera may be used as a device for reading a document or similar text. The prerequisite with this kind of application is that the decrease in the quantity of light in the peripheral portion of an image be corrected; otherwise, characters would be illegible in the peripheral portion when reproduced, failing to transfer information accurately.